A Linux desktop environment is a collection of applications designed to work well with each other and provide a consistent user experience. A desktop environment is usually paired with a login manager. The login manager, also known as a greeter or display manager is started automatically at system initialization. The login manager is an X Windows (or Wayland) application to help users log in locally or remotely and possibly choose a different desktop environment. The login manager desktop environments typically include one or more panels, an application launcher, a file manager, a settings manager, and a compositing window manager all with a consistent look and feel.
Link: Learn Linux 101: Graphical desktops